The present invention relates to a zoom lens which can be used in an inner focus type video camera.
Recently, an inner focus type zoom lens has been developed and put to practical use. In this type, an element lens such as a compensator or a master lens is shifted minutely along the optical axis for focusing. One example of such an inner focus type zoom lens, used mainly for a video camera, is constituted by four lens groups including an aspherical lens to reduce the number of constituent lenses. (A similar zoom lens is disclosed in Japanese Patent Provisional Publication No. Hei2-39011.)
In the conventional inner focus type zoom lens, aspherical surfaces are formed by glass mold, or by complex aspherical surfaces. However, aspherical surfaces are difficult to manufacture and increase the manufacturing cost. Because of this cost increase, the conventional zoom lens is unsatisfactory in cost reduction as a whole, despite the reduction in the number of the required lenses. The use of one or more plastic lens is effectual for cost reduction. This measure for cost reduction is difficult, however, since there are restriction of the refractive index and Abbe number, difficulties in forming process, and a problem of a focus displacement due to a temperature change.